Question

Give a description of how a sorption isotherm is obtained for a food. Describe three factors that can affect water activity in a food. Give three examples of types of food deterioration affected by water activity and indicate what values of water activity are suitable for minimizing each kind of deterioration.

Answer 1

Sorption Isotherm is a plot of set of data which shows relation between moisture content and
relative humidity at constant temperature and equilibrium conditions.
Determining Sorption isotherm for food :
First of all we create a closed chamber having constant relative humidity and temperature where
we also place a salt solution. Also the food sample whose moisture content is to be determined
is also placed in the chamber, over the salt solution on a stand and is allowed to sit there for
certain period of time. If the sample is dry and humidity is high it will absorb. If the sample is
moist and the humidity is low it will desorb. Now you can use different salt solutions with varying
relative humidities to find their corresponding moisture content at equilibrium using this
experimentation process.
This is how you obtain data and plot it to get sorption isotherm as shown below



3 Factors that can affect water activity in a food:
Temperature, solubility & moisture content

3 examples of types of food deterioration affected by water activity:
(i) microbial spoilage (ii) Color changes/chemical changes (iii) spoilage in flavor and odor
Values of a_w which are suitable for minimizing each kind of deterioration stated above,
(i) microbial spoilage : for bacterial spoilage a_w should be below 0.91 and to prevent the growth
   of mold it should be below 0.81
(ii)Color changes/chemical changes : Foods containing proteins and carbs tend to brown.
higher a_w indicates that it is more likely to brown and react. So, the a_w should be lowered
   ( the less, the better !)
(iii) spoilage in flavor and odor : This is caused due to some enzymatic reactions again, a_w should
   be maintained below 0.8